It was previously shown that a number of plant growth promoting rhizobacteria contain an enzyme, 1-aminocyclopropane-1-carboxylate deaminase, that catalyses the cleavage of 1-aminocyclopropane-1-carboxylate, the immediate precursor of ethylene in plants. Moreover, experimental evidence indicated that the activity of this enzyme was the key factor in the ability of plant growth promoting rhizobacteria to stimulate the elongation of plant roots. In the model presented in this manuscript we address the question of how the bacterial enzyme 1-aminocyclopropane-1-carboxylate deaminase, with a low affinity for 1-aminocyclopropane-1-carboxylate, can effectively compete with the plant enzyme 1-aminocyclopropane-1-carboxylate oxidase, which has a high affinity for the same substrate, 1-aminocyclopropane-1-carboxylate, with the result that the plant's endogenous ethylene concentration is reduced. It is argued that the simplest explanation for the observed biological activity of plant growth promoting rhizobacteria relates to the relative amounts of 1-aminocyclopropane-1-carboxylate deaminase and 1-aminocyclopropane-1-carboxylate oxidase in the system under consideration. For plant growth promoting rhizobacteria to be able to lower plant ethylene levels, the 1-aminocyclopropane-1-carboxylate deaminase level should be at least 100- to 1000-fold greater then the 1-aminocyclopropane-1-carboxylate oxidase level. This is likely to be the case, provided that the expression of 1-aminocyclopropane-1-carboxylate oxidase has not been induced.Copyright 1998 Academic Press Limited Copyright 1998 Academic Press Limited